This invention relates to a process for preparing N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid using a homogeneous catalyst system. More particularly, this invention relates to a process for producing N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid using a salt of cobalt or manganese in the presence of bromide ions.
N-phosphonomethylglycine, known in the agricultural chemical art as glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation and aquatic plants. N-phosphonomethylglycine and its salts are conveniently applied in an aqueous formulation as a post emergent phytotoxicant for the control of numerous plant species. N-phosphonomethylglycine and its salts are characterized by broad spectrum activity, i.e., the controlled growth of a wide variety of plants.
Numerous methods are known in the art for the oxidation of N-phosphonomethyliminodiacetic acid to N-phosphonomethylglycine. For example, U.S. Pat. No. 3,969,398 to Hershman discloses a process for the production of N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid utilizing a molecular oxygen-containing gas as the oxidant in the presence of a catalyst consisting essentially of activated carbon. U.S. Pat. No. 3,950,402 discloses a method wherein N-phosphonomethyliminodiacetic acid is oxidized to N-phosphonomethylglycine in an aqueous media using a free oxygen-containing gas and a noble metal catalyst, such as palladium, platinum or rhodium on a support. U.S. Pat. No. 3,954,848 discloses the oxidation of N-phosphonomethyliminodiacetic acid with hydrogen peroxide and an acid such as sulfuric or acetic acid. Hungarian Patent Application No. 011706 discloses the oxidation of N-phosphonomethyliminodiacetic acid with peroxide in the presence of metals or metal compounds.
There are many references in the literature on the oxidation of alkylaromatic hydrocarbons to aromatic acids in the presence of various catalyst systems. For example, Sheldon and Kochi, Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, New York (1981), pages 126-129, discloses that bromide, as hydrogen bromide, sodium bromide, or organic bromide, has a pronounced synergistic effect on the cobalt-and manganese-catalyzed autoxidations of alkylaromatic hydrocarbons.
Although satisfactory results are achieved by the above prior art processes for oxidizing N-phosphonomethyliminodiacetic acid to N-phosphonomethylglycine using heterogeneous catalysts such as activated carbon or platinum deposited on activated carbon, or the use of an acid catalyzed reaction such as the use of sulfuric acid, there is still a need for improved processes to prepare N-phosphonomethylglycine in high yields, which minimizes the formation of undesirable byproducts, such as phosphate.